The present invention relates to an improvement in a molding method of a disc, particularly, a plastic disc for a high-density information recording carrier such as a video disc, an optical disc and the like, and more particularly an improved metal mold and a stamper to be used for an injection molding of the plastic disc.
In general, a plastic disc to be used for a video disc or an optical disc is very thin and flat such that a thickness of the plastic disc is about 1.2-1.5 mm. The diameter is about 300 mm maximum. Acrylic resin (PMMA) and polycarbonate resin (PC) having low fluidity are used as materials for the plastic disc. Therefore, it is difficult to mold a completely satisfactory plastic disc even at present. In various performances required by such kind of disc, stability in size and shape of the molded product is important. For example, a thickness of the disc must be 1.2.+-.0.1 mm, and there must not be any curve and twist in the molded product. Even when an ultra precision injection molding machine which is being now developed is used, it is difficult to mold such a super precise plastic disc. A primary reason of such difficulty is that when molten resin is uniformly injected and filled into a mold cavity having a very small lateral width and a large diameter, a difference in solidification speed of the resin and in filling pressure is created between a position near a gate and a position away from the gate. This results in ununiformity of a thickness of the molded product, and further occurrence of curve and twist of the product due to residual distortion.
Although PMMA and PC having good flowability and low molecular weight may be used as resin materials, so as to obviate the above-mentioned defects, it is necessary to use PMMA and PC having low fluidity and high molecular weight to enhance durability, heat-resistance and dimensional stability of the disc. However, in the case that such materials having high molecular weight are used, transferability or replicating property from the stamper as well as stability in size and shape deteriorate.
A method of producing a disc for a high-density information recording carrier by injection molding as directed by the present invention is known in the art, e.g. Japanese Patent Laid-Open Patent Appln. No. 56-139940 and Japanese Patent Laid-Open Patent Appln. No. 57-203517. In these injection molding methods, size of a molding cavity of a metal mold is set in such that a shrinkage rate of a desired molded product is included. However, in practice, the thickness of the plastic disc molded by using this metal mold does not become identical with the size of the molding cavity, but a central portion of the disc becomes relatively thicker as compared with an outer peripheral portion thereof, and further dimensional stability of the molded product is deteriorated because of the shrinkage.